1. Field of the Invention
The present invention relates to a hydropower conversion method and system, and in particular, to a system and method of using a hydraulic gradient to create a temporary negative pressure for use in operating a pneumatic device.
2. Background of the Invention
Generation of electrical energy is needed to support the world""s growing population and economy. Personal, commercial and defense energy demands are currently taxing the existing electrical energy supply. To meet the electrical energy demand, there has been great interest in exploiting renewable energy resources, such as hydroelectric power. Hydroelectric power is seen as an energy source which will reduce dependence on foreign energy and avoid potentially costly and possible negative environmental effects linked to non-renewable energy production technology. Examples of some environmental concerns include carbon dioxide emissions, acidic mine drainage from coal extraction waste, heat disposal and radioactive fission products produced during nuclear energy generation.
Hydroelectric energy has been seen as an alternative electrical energy generation system that has few potentially negative environmental effects. However, dams used in conventional hydroelectric power systems have been linked to negative physical, chemical and biological effects on the bodies of water to which these dams are disposed. These negative environmental effects manifest themselves in habitat destruction, obstructions to natural fish movement, poor water quality, over harvest and competition from non-indigenous species. Further, hydroelectric dams may degrade riverine habitat and impede movement of migratory fishes to and from their natal streams.
In a conventional hydroelectric power system, potential energy is used to accelerate water in a discharge line (i.e., penstock) of variable length, geometry and slope. The accelerated water is then directed through a turbine assembly designed to convert the kinetic energy of the water into mechanical energy. The gross head, H, available for acceleration is the elevation differential between the forebay water surface and the tailwater surface as provided by the formula:
Gross Head (ft)=(Elevationforebay)xe2x88x92(Elevationtailwater)xe2x80x83xe2x80x83(1)
The energy potential represents the product of gross head and the volumetric flow rate of water through the system as provided by:
Energy Potential (kWh)=306.6.xe2x80xa2Qxe2x80xa2Hxe2x80x83xe2x80x83(2)
where
Q=water flow rate (ft3/s)
H=gross head (ft).
For effective use, H has a minimum value of 10 feet. As H drops towards the minimum acceptable value of 10 feet, the water velocities achieved become unacceptably low so as to be impractical for use in hydroelectric generation. Implementation of a conventional hydroelectric system with these low water velocities would require the employment of a large diameter, slowly rotating turbine design. Such a design would involve an excessive capital investment to deploy.
Bernoulli""s equation applied to the forebay and the tailwater conditions allows calculation of the maximum water velocity (Vmax) in the turbine inlet as                                           (                                          P                γ                            +              Z              +                                                V                  2                                                  2                  ⁢                  G                                                      )                    forebay                =                              (                                          P                γ                            +              Z              +                                                V                  2                                                  2                  ⁢                  G                                                      )                    tailwater                                    (        3        )            
where
P=pressure (psig)
xcex3=specific weight of water (lb/ft3)
V=velocity (ft/s)
G=gravity (32 ft/s2)
Z=elevation head (ft)
Since P and V at both locations are negligible and neglecting minor line losses (friction), Vmax is
Vmax(ft/s)={square root over ((2g(Z1xe2x88x92Z2))}.xe2x80x83xe2x80x83(4)
For example, with H defined as Z1xe2x88x92Z2, of 10 feet, Vmax is 25.4 ft/s whereas an H of 100 feet provides a Vmax of 80.2 ft/s. Hence, for a given volumetric flow rate (Q), the penstock cross-sectional area required at H=10 feet is 3.16 times that required at H=100 feet. The lower water velocity also results in reduced turbine tip speeds and thus increases turbine shaft requirements, e.g., costs, for a given power output.
Torque applied to the turbine shaft determines the diameter of the turbine shaft. Further, the turbine shaft diameter is directly related to power and shaft speed:                               torque          ⁢                      xe2x80x83                    ⁢                      (                          pound              ⁢                              -                            ⁢              inches                        )                          =                              63025            ·            HP                    RPM                                    (        5        )            
where
HP=horsepower
RPM=shaft speed (revolutions per minute).
Unfortunately, large values of H result in problems associated with greater forebay volumes and the need for equipment that allows for the movement of fish both upstream and downstream. Excessive shear, turbulence, flow separation, and gas supersaturation are also of primary concern.
A disadvantage with the design of conventional hydroelectric power generation systems is that these systems are unable to produce power under low-head conditions (i.e., less than ten feet) economically or without potentially negative environmental effects. Examples of such hydroelectric power designs include Pelton, Francis, Turgo, Kaplan and cross-flow designs. Due to the inability of these conventional systems to function under low-head conditions, several alternative designs have been developed for low-head applications.
For example, in EPO Patent 0,117,739 to Smith, there is disclosed a water engine design having reciprocating floats in vertical chambers that rise and fall with water levels as directed by inlet and exhaust valves. Power is transmitted by oscillation of a pivotally mounted beam assembly.
In U.S. Pat. No. 4,782,663 to Bellamy, there is disclosed a pneumatic hydroelectric power conversion system. Power is generated by passing water in sequence over flexible bags or membranes to displace air under the flexible bags. Power is developed by directing the displaced air through an air turbine coupled with a generator.
In U.S. Pat. Nos. 5,074,710, 4,095,423, 4,103,490 and 4,464,080, all to Gorlov, there are disclosed various apparatus that include one or more vertical chambers with parts of ingress and egress through which tidal or river flows are directed so as to alternately force air out and into the chambers via an air motor or a turbine.
In EPO Patent 0,339,246 to Loughridge, there is disclosed a power generation system similar to that of Gorlov but designed so as to introduce water alternately into one of two columns thereby tangentially creating a swirling action that minimizes hydraulic losses.
In U.S. Pat. No. 4,288,985 to Dyck, there is disclosed an apparatus that uses two reservoirs influenced by tidal action to force water back and forth through an air tight duct system in which a turbine is mounted for producing power.
In U.S. Pat. No. 5,377,485, EPO Patent 0,526,470 B1, and PCT Patent WO 91/17359, all to Bellamy, there are disclosed various power conversion systems that direct water through a duct and as a result, the water induces air into the duct. The induced airflow exits the duct through an exhaust duct and is drawn through an air turbine. The air is introduced into the duct by siphon, air injectors or venturis.
In EPO Patent 0,100,799 to Cary, there is disclosed an apparatus that makes use of a descending column of water to entrain and compress air on a continuous basis. The air is separated from the water at a particular depth through the use of a tangentially fed air inclusion chamber. The compressed air is then used to supply air to a gas turbine or ramjet.
In EPO Patent 0,162,814 to Burgnoli, there is disclosed an apparatus that improves the efficiency of a hydraulic air compressor, such as that described in the Cary patent, by recirculating air that is under negative gauge pressure as the air exits an air motor designed to produce power.
In U.S. Pat. No. 4,098,081 to Woodman, there is disclosed a power generation system that uses a plurality of tidal chambers which are filled in succession during rising tide and which are allowed to sequentially empty during falling tide. Air flows across a turbine as a manifold and a valve means communicates air pressure and vacuum from the tidal chambers.
The systems developed thus far generally have one or more of the following disadvantages; too complex, require excessive capital, create undesirable levels of gas supersaturation in water, lack efficient means of converting low pressure air flows into power, require materials that have yet to be developed or have low energy production potentials so as to not be viably practical as a hydroelectric power system. Therefore, in spite of the recently developed designs, there is still a need for an efficient and economical low-head power conversion system that minimizes damage to the environment.
For example, there is a need to reduce impacts on fishery resources through application of improved hydropower technologies, specifically those that reduce required dam height, eliminate turbine blade induced mortality of migrating fishes and limit or prevent supersaturation of water with air which has been linked to gas bubble disease of aquatic species. Further, there is need for an economical method that converts low grade (low head) energy potentials into high grade (high air pressure differential) energy sources so as to minimize the scale and cost of the energy conversion equipment.
In accordance with the present invention, a hydropower conversion system is provided which uses a hydraulic gradient to accelerate water in a drive pipe thus converting potential energy into kinetic energy. A closure device, such as a valve, is positioned downstream of an inlet of the drive pipe to interrupt water flow in the drive pipe, thus creating a temporary negative pressure down stream of the valve. A negative pressure relief device is coupled to the drive pipe and is positioned near the valve. The negative pressure relief device includes a riser section which allows the negative pressure to pull gas (e.g. air) but not water into the riser section when the pressure relief device is under negative pressure (i.e. a vacuum). A standing column of water in the riser section assists in pulling the gas into the riser section.
Gas under vacuum powers a turbine or other pneumatic equipment including water lifters (i.e. pumps). A timer, pressure or mechanical linkage is provided to open the drive pipe valve to reestablish water flow in the drive pipe to thereby allow a repeat of the power generation steps. The reestablished water flow and pressure is used to force gas out of the riser section and into the atmosphere or to recovery equipment operable by gas flow. A vacuum tank may be used to store energy as gas under vacuum.
In varying embodiments, two or more independent hydropower conversion systems may operate in series or parallel to reduce variations in the vacuum tank gas flow. In addition, air expansion in the hydropower system may be used to establish a refrigeration potential.
An object of the present invention relates to providing hydroelectric power or an inexpensive water pumping potential using previously untapped low head resources such as rivers and tidal flows.
Another object of the present invention relates to providing a hydroelectric power system which can be adapted to capture additional energy from the tailwaters from large conventional hydroelectric and coal fired power systems.
A further object of the present invention relates to providing a hydroelectric power system which does not negatively impact aquatic resources.
An additional object of the present invention relates to providing a hydroelectric power system for use by a single user or residence to supplement other methods of on-site energy production such as wind, solar, and geothermal.
Yet another object of the present invention is to provide a hydroelectric, water pumping, and refrigeration system for use in underdeveloped regions of the world.
A further object of the present invention relates to providing a hydroelectric power system which may be adapted for use as part of a localized or widespread distributed system of diverse electric power generation sources.
According to one aspect of the present invention, a hydropower conversion system is provided which uses a hydraulic gradient to accelerate a flow of water. A drive pipe has an inlet and an outlet for communicating the water flow. A valve is disposed downstream from the inlet for interrupting the water flow into the drive pipe to thereby create a temporary negative pressure downstream from the valve. A negative pressure relief device includes a riser section coupled to the drive pipe downstream from the valve. The negative pressure relief system is for pulling gas but not water into the riser section when the negative pressure relief device is subject to negative pressure. A pneumatic device is coupled to the riser section of the negative pressure relief device and powered by the negative pressure.
According to another aspect of the present invention, a method is provided for hydropower conversion using a hydraulic gradient. The method comprises using the hydraulic gradient to accelerate a quantity of water into an inlet of a drive pipe. The flow of water is interrupted in the drive pipe to create a temporary negative pressure in the drive pipe. Gas is pulled into a pressure relief device coupled to the drive pipe, without introducing water into the pressure relief device, using the negative pressure. A pneumatic device is coupled to the pressure relief device and is powered from the negative pressure.
A key feature of the present invention concerns the capability to generate hydropower energy efficiently at hydraulic heads that are well below the minimum required to support conventional hydroturbine designs and to achieve this result without the need for water to pass through rotating turbine blades. Potential water sources with which the present invention may be employed include free flowing streams, channel courses and tidal flows.
An advantage of minimizing head requirements is the increase in the number of potential application sites while, in some cases, eliminating the need for permanent water control structures such as dams. The elimination of the need for water to pass through rotating turbine blades obviates excessive shear, strike and pressure changes known to harm entrained fish and other forms of aquatic life. Therefore, an additional advantage of the present invention concerns circumventing the problem of excessive shear, turbulence, flow separation and gas supersaturation by developing equipment capable of economically recovering energy from water in low head applications.
An additional feature of the present invention concerns the use of power converted above flood stages to minimize construction costs. The air turbine generator assembly and control system can be located out of the river channel, e.g. on high ground, thereby reducing construction costs. Further, a single air turbine generator assembly could serve several sites thereby allowing for centralized power production.
Another feature of the present invention concerns the use of the vacuum developed during operation to provide for gas cooling without the need for conventional inefficient refrigeration cycles. Expansion of gas from atmospheric pressure to subatmospheric pressure will result in gas cooling due to the Joule-Thompson effect. Consequently, the expansion of gas could be used as an alternative to conventional, inefficient refrigeration cycles based on freon or ammonia gas.
A further feature of the present invention concerns the ability to operate with entrained particulate matter such as sand that impairs performance of conventional hydroelectric power systems. As a result, pretreatment costs are minimized.
An additional feature of the present invention concerns the control of vacuum pressure generated so as to maintain pressure differentials that allows for economical energy recovery while also minimizing effects on entrained fish.
Yet a further feature of the present invention concerns the generation of high levels of vacuum. These high levels of vacuum allow for the use of small, cost effective, air motors and turbines that spin at speeds equal to, or greater than, the 60 Hz power line frequency. In addition, these high levels of vacuum allow for the direct suction lift of water to levels tens of feet above the water source. This eliminates the need for motors, pumps and power control systems used in conventional water pumping stations thereby providing for a savings in cost and an increase in energy transfer efficiency.
An additional feature of the present invention relates to storing energy for later use by creating a vacuum in tanks or gas tight underground structures. This is feasible as a result of the unique and high levels of vacuum possible by use of the invention.
An additional feature of the present invention relates to using the natural slope of a flowing water source to establish the hydraulic gradient. An advantage of this modification is provided in the elimination of the need for a dam, thereby reducing environmental effects and construction costs associated with dams employed in conventional hydroelectric power systems.
Yet another feature of the present invention relates to using tidal flows of a water source to establish the hydraulic gradient. As with the use of the natural slope of a flowing water source, an advantage of this modification is provided in the elimination of the need for a dam as well as being able to use previously untapped water sources to generate power.
An additional advantage of the present invention concerns the elimination of a rotating turbine blade assembly in direct contact with flowing water found in most conventional hydropower generation systems. As a result, there is a major cost saving provided by the present invention over those conventional systems which require a very large and slowly rotating assembly. A further advantage is provided by the elimination of blade induced mortality in fish resulting from strike, shear, turbulence and pressure forces possible in conventional systems.
A further advantage of the present invention concerns the ability to operate with a minimal hydraulic gradient which, in many cases, eliminates the need for dams thus circumventing the problems associated with sediment build-up, fish movement, water temperature and costs associated with dams. However, if a dam is required, the dam height requirement will be within the range of low-head inflatable dam structures available commercially at relatively low cost.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.